This invention is directed to an improved high density, backplane connector of the type taught in U.S. Pat. Nos. 4,969,824 and 5,171,154, incorporated herein by reference, and assigned to the assignee hereof. The prior art generally describes a pair of one-piece connector housing members, arranged for mounting to a first planar electronic device, such as a motherboard, where such housing members are spaced apart to define a slot for receiving a second planar electronic device, such as a daughter board, to be electrically interconnected to said mother board. In a commercial embodiment of the invention, the respective housings faces, adjacent the planar electronic devices, include channels for receiving a coiled spring, and a flexible film, having electrical circuitry or traces thereon, wrapped about the housing members and coiled springs. The coiled springs provide a normal force to the film to ensure electrical contact with the respective planar electronic devices.
Experience has shown that connectors of this type may exhibit some difficulties with longevity in terms of the number of insertions by the daughter board, as well as reliability of making good electrical connections. One of such difficulties is associated with the nature of the flexible film, and the copper circuitry thereon. Such difficulty is addressed in the co-pending application, Ser. No. 08/292,221 dated Aug. 31, 1994, filed concurrently with this application. For a detailed discussion of the problems overcome in the co-pending application, reference may be made thereto, where such co-pending application is incorporated herein in its entirety. Briefly stated, there is a tendency for the copper circuitry or traces to work harden resulting in stress cracking and/or flaking thereof leading to poor or failed electrical continuity.
In any case, with the type of connector of this invention, as the daughter board is pushed into position within the slot created by the opposed housing members, its front edge deflects the flexible film towards the housing. As the daughter board moves incrementally into position adjacent the mother board, it will encounter the mechanical resistance associated with the compression of the first coil spring in the assembly. This will create a local sharp deformation of the foil, an event which will repeat as many times as there are coil springs in the assembly. Simultaneously, the distance from the center of any pad or trace to the mother board will decrease somewhat due to the gradual assumption of a straight line shape of the flexible circuit. This movement of the buckling wave starting with the first wave until the end, creates stress fatigue and cracking of the copper conductors, which of necessity are made of relatively soft copper, but which work harden.
While the co-pending application addresses the problem of local compliance, i.e. compliance between adjacent conductors or traces in close proximity to one another, the fundamental problem of the buckling wave is not completely eliminated. The present invention thus proposes a connector construction which will satisfy the fundamental requirement of keeping the flexible film under tension so that the frictional force of card insertion exerted on the flexible film would be smaller than the tension force. Thus, when the daughter board is inserted and compression of the connector is taking place, the "extra length" of the flexible film is accommodated by a motion of the flexible film in the direction opposite that of the daughter board insertion. In this manner, the flexible film will not be forming a buckling wave and therefore the conductors will not be subjected to fatigue stress cracking. The present invention accomplishes this by having the respective housings of the connector made in two pieces with a spring or separating means to counteract the force of insertion. The advantages of the present invention will become apparent to those skilled in the art from a reading of the following specification, particularly when read in conjunction with the accompanying drawings.